1. Field of the Invention
This invention relates to a semiconductor ceramic composition having a positive temperature coefficient of resistivity, which is used for a PTC thermistor, a PTC heater, a PTC switch, a temperature detector, and the like.
2. Description of the Related Art
Conventionally, as materials showing a PTCR characteristic (Positive Temperature Coefficient of Resistivity), there have been proposed compositions in which various semiconductive dopants are added to BaTiO3. These compositions have Curie temperatures around 120° C. Depending on the use, these compositions need to be shifted in the Curie temperatures.
For example, it has been proposed to shift the Curie temperature by adding SrTiO3 to BaTiO3; however, the Curie temperature is shifted only in a negative direction and is not shifted in a positive direction in this case. Currently, only PbTiO3 is known as an adding element for shifting the Curie temperature in a positive direction. However, since PbTiO3 includes an element that causes environmental pollution, a material which does not use PbTiO3 has been in demand in recent years.
In the BaTiO3 semiconductor porcelain, with an object of preventing a reduction in temperature coefficient of resistivity by Pb substitution as well as promoting productivity or reliability by reducing a voltage dependency, there has been proposed a method of producing a BaTiO3 semiconductor porcelain in which one or more kinds of Nb, Ta, and a rare earth element is added to a composition obtained by using no PbTiO3 and controlling x to a range of 0<x≦0.15 in a structure of Ba1−2×(BiNa)×TiO3 that is achieved by substituting a portion of Ba of BaTiO3 by Bi—Na; followed by sintering the composition in nitrogen; and thereafter subjecting the composition to a heat treatment in an oxidation atmosphere (see Patent Reference 1).                Patent Reference 1: JP-A-56-169301        